A light-emitting diode (LED) can provide light in a more efficient manner than an incandescent and/or a fluorescent light source. The relatively high power efficiency associated with LEDs has created an interest in using LEDs to displace conventional light sources in a variety of lighting applications. For example, in some instances LEDs are being used as traffic lights and to illuminate cell phone keypads and displays.
Typically, an LED is formed of multiple layers, with at least some of the layers being formed of different materials. In general, the materials and thicknesses selected for the layers influence the wavelength(s) of light emitted by the LED. In addition, the chemical composition of the layers can be selected to promote isolation of injected electrical charge carriers into regions (e.g., quantum wells) for relatively efficient conversion to light. Generally, the layers on one side of the junction where a quantum well is grown are doped with donor atoms that result in high electron concentration (such layers are commonly referred to as n-type layers), and the layers on the opposite side are doped with acceptor atoms that result in a relatively high hole concentration (such layers are commonly referred to as p-type layers).
In order to produce desired light, wavelength converting materials such as phosphors is disposed over the light emission surface of an LED. This technique has been used to produce white light from a single diode. Yellow-white light-emitting LED is produce by coating an LED emitting light of a wavelength of 470 nm (blue light) with a yellow emitting phosphor. However the resultant white light does not have the necessary color temperature to be used in most general lighting applications.
When white light-emitting devices are used for general lighting applications, a high color rendering white light may be desired. Generally, color rendering refers to how natural the colors of objects look under a given illumination and can be an important characteristic for light sources for general lighting. Color rendering is measured by the general Color Rendering Index (CRI). As used herein, the general color rendering index refers to a method for describing the effect of a white light source on the color appearance of objects. Specifically, Ra is a measure of the average appearance of eight standardized colors chosen to be of intermediate saturation and spread throughout the range of hues. Luminous efficacy is the ratio of luminous flux (lumen) emitted by the source to the input electrical power (watt). Color Temperature (CT) is the appearance of light emitted from a white light source. It defines how warm or how cold a certain white light may appear. In addition to a high color rendering, a good luminous efficacy may be desired, as well as a color temperature that represents the desired lighting application. A white light that contains all these attributes is considered a color balanced white light. Different amounts of each attribute may be needed for different lighting application such as indoor or outdoor lighting, office or home lighting.